Nuclear radiation gauges have been widely used for measuring the density of soil and asphaltic materials. Such gauges typically include a source of gamma radiation which directs gamma radiation into the test material, and a radiation detector located adjacent to the surface of the test material for detecting radiation scattered back to the surface. From this detector reading, a determination of the density of the material can be made.
These gauges are generally designed to operate either in a "backscatter" mode or in both a backscatter mode and direct transmission mode. In gauges capable of direct transmission mode, the radiation source is vertically moveable from a backscatter position, where it resides within the gauge housing, to a series of direct transmission positions, where it is inserted into small holes or bores in the test specimen.
Many of the gauges commonly in use for measuring density of soil, asphalt and other materials are most effective in measuring densities of materials over depths of approximately 4-6 inches. However, with the increase in cost of paving materials, the practice in maintaining and resurfacing paved roadbeds has become one of applying relatively thin layers or overlays having a thickness of one to three inches. With layers of such a thickness range, many density gauges are ineffective for measuring the density of the overlay because the density reading obtained from such gauges reflects not only the density of the thin layer, but also the density of the underlying base material.
Nuclear gauges capable of measuring the density of thin layers of materials have been developed by the assignee of the present invention. For example, thin layer density gauges are disclosed in U.S. Pat. Nos. 4,525,854, 4,701,868, and 4,641,030, all of which are assigned to the assignee of the present invention and are incorporated herein by reference in their entirety. The gauges disclosed in the above-referenced patents are referred to as "backscatter" gauges because the radiation source does not move outside the gauge housing, which is necessary for measurement in the direct transmission mode.
As disclosed in the above patents, the preferred method of measuring the density of thin layers of materials, such as asphalt, requires two independent density measurement systems. The geometry of these two measurement systems must be configured with respect to one another and with respect to the medium being measured in such a manner that they measure two different volumes of material. The two different volumes are not mutually exclusive insofar as they partially overlap one another. Measurement accuracy depends upon a larger portion of the volume measured by one of the measurement systems being distributed at a lower depth beneath the gauge than the volume measured by the other measurement system. This is accomplished by placing one radiation detection system in closer spatial proximity to the radiation source than the other detection system.
There remains a need in the art for a nuclear gauge capable of operating in both backscatter mode and direct transmission mode, and which is suitable for measuring the density of thin layers of material.